The present invention relates broadly to thermoplastic polyurethane elastomers, and more specifically to a p-phenylene diisocyanate (PPDI)-based composition therefor having a unique chain extender combination for improved injection moldability.
Polyurethane elastomers are often specified for applications wherein a material is specified as having exceptional resistance to abrasion, solvents, and oxygen aging, in addition to superior toughness, tear strength, elongation, shock absorption, hardness, flexibility, elasticity, and dynamic properties. Interest in these materials has continued to increase with the introduction of injection-moldable grades which may be economically processed for use in a wide variety of industrial and consumer products including: gears, bearings, and joints for precision machines; parts for electronic instruments; soles and uppers for atheletic shoes and ski boots; automotive parts; and seals, gaskets, and packings for hydraulic fluid systems and other applications.
Thermoplastic polyurethane materials, and particularly the injection-moldable TPUs of the type herein involved may be prepared by reacting diols or polymeric polyols, diisocyanate compounds, and polyfunctional chain extender compounds having, for example, hydroxyl or amino functional groups. Different combinations of these reactants have been processed by various methods to yield TPUs having a diverse range of properties. For example, U.S. Pat. Nos. 4,371,684 and 4,245,081 discloses a continuous process for the production of TPUs involving the steps of reacting in an extruder a linear polyol, a diisocyanate, and a chain extender which is a mixture of at least two different glycols. Preferred linear polyols include polyester, polycaprolactone, polyether, polythioether, polyesteramide, polycarbonate, and polyacetal polyols, with preferred diisocyanates including aliphatic, cycloaliphatic, aromatic, araliphatic and heterocyclic diisocyanates generally known in the art such as diphenyl methane diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, lysine ester diisocyanates, and tolylene, diphenyl methane, and xylene diisocyanates and hydrogenation products thereof. The preferred glycols include ethylene glycol, di- and tri-ethylene glycol, 1,2-propane diol, 1,3- and 1,4-butane diol, 1,6-hexane diol, 2-ethyl-1,3-hexane diol, 2,2-dimethyl propane diol, 1,4-bis-hydroxylmethyl cyclohexane, and hydroquinone dihydroxy ethyl ether. The chain extender mixture is stated to improve the processing of the composition in a twin- or other multi-screw extruder. The reference is silent, however, as to whether the chain extenders improve the injection molding properties of the resulting TPU.
U.S. Pat. No. 5,066,762 discloses a TPU resin having improved hydrolysis and heat deterioration resistance, temperature dependency, and compression set. The TPU is prepared by the reaction of p-phenylene diisocyanate, a hydroxyl terminated poly(hexamethylene carbonate) polyol, and a short chain polyol, such as ethylene glycol, 1,3-propylene glycol, 1,4-butanediol, neopentyl glycol, 3-methyl-1,5-pentanediol, p-xylene glycol, 1,4-bis-(xcex2-hydroxyethoxy)benzene, 1,3-bis-(xcex2-hydroxyethoxy)benzene, cyclohexane 1,4-dimethanol, octane-1,8-diol, and decane-1,10-diol, as a chain extending agent.
Castro, Hentschel, Brodowski, and Plummer, xe2x80x9cInfluence of Processing Conditions on Mechanical Properties of High Performance CHDI Based TPUs,xe2x80x9d J. Elast. and Plast., Vol. 17 (October 1985), pp. 238-248, discloses that mixed chain extenders may be employed to facilitate the injection molding of a trans-1,4-cyclohexane diisocyanate (CHDI)-based TPU. The chain extenders employed were bis (2-hydroxyethyl) bisphenol A, i.e., Dianol 22, and cyclohexane dimethanol (CHDM). A p-phenylene diisocyanate (PPDI)-based formulation additionally is referenced as illustrative of another high performance TPU.
Hepburn, xe2x80x9cPolyurethane Elastomers,xe2x80x9d 2d ed., pp. 249-280, Elsevier Applied Science (London and New York, 1992), discloses the use of mixed chain extender systems in CHDI- and PPDI-based TPUs formulated with polycaprolactone (Capa 225) as the polyol. Bi-component combinations of 1,4-butane diol, 1,4cyclohexane dimethanol, 1,6-hexane diol, 1,1xe2x80x2-isopropylidene-bis-(p-phenylene-oxy)-di-xcex2-ethanol (Dianol 22), 1,1xe2x80x2-isopropylidene-bis-(p-phenylene-oxy)-di-xcfx89-propanol (Dianol 33) are employed for the chain extender system. Additionally disclosed is a PPDI-based TPU formulated with polytetramethylene glycol as the polyol which uses as a single-component chain extender such as hydroquinone dihydroxy ethyl ether.
U.S. Pat. No. 4,980,445 discloses a TPU exhibiting high resistance to abrasion forces while retaining a good impact resistance and creep behavior. The TPU is formed from the reaction product of a mixture of a polyester and a polyether diol, a difunctional chain extender, and a diisocyanate. The polyester diol may be a poly(oxycaproyl) diol or a poly(alkylene alkanedioate)diol, such as a poly(ethylene adipate), poly(propylene adipate), poly(butylene adipate), poly(ethylene/butylene adipate) diol, while the polyether diol may be a poly(alkylene oxide) glycol such as poly(ethylene oxide) diol, poly(propylene oxide) diol, poly(tetramethylene oxide) diol, or a block or random polyoxypropylene/polyoxyethylene or polyoxytetramethylene/polyoxyethylene copolymeric glycol. Preferred difunctional chain extenders include ethylene and propylene glycol, 1,4-butane diol, 1,3-butane diol, 1,5-pentanediol, 1,6-hexanediol, 1,10-decanediol, 1,12-dodecanediol, ethoxylated hydroquinone, 1,4-cyclohexane diol, N-methylethanolamine, N-methylisopropanolamine, 4-aminocyclo-hexanol, 1,2-diaminoethane, 2,4-toluenediamine, and the like. Preferred diisocyanates include aromatic, aliphatic, and cycloaliphatic diisocyanates such as m- and p-phenylene diisocyanates, 2,4- and 2,6-toluene diisocyanate, 4,4xe2x80x2-methylenebis(cyclohexyl isocyanate), and mixtures thereof The TPU may be prepared in a single-step process wherein the diols, diisocyanate, and chain extender are simultaneously mixed and reacted at an elevated temperature, or in a two-step process wherein the mixture comprising the diols is first reacted with the diisocyanate to form a diisocyanate-terminated prepolymer, which prepolymer is further reacted with the difunctional chain extender. Alternatively, the diisocyanate may first be reacted with the chain extender to form a product which is then reacted with the polyester and polyether diol mixture.
U.S. Pat. No. 5,013,811 discloses a linear TPU elastomer of a polyol, a first and second chain extender, and a diisocyanate component. The polyol may be a polyether polyol, polycarbonate polyol, polycaprolactone polyol, polyester polyol, polybutadiene polyol, or a mixture thereof The diisocyanate preferably is a diphenylmethane diisocyanate (MDI). The first and second chain extenders may be selected as different polyol or amine compounds having a molecular weight of less than about 500. Preferred first and second chain extends include 1,4-butane diol, tripropylene glycol, dipropylene glycol, propylene glycol, ethylene glycol, 1,6-hexane diol, 1,3-butane diol, neopentyl glycol, ethylene diamine, and mixtures thereof The TPU is formed at a relatively low processing temperature by initially reacting the diisocyanate with the first chain extender to form a modified diisocyanate, which diisocyanate is then reacted with the polyol and second chain extender.
U.S. Pat. No. 3,929,732 discloses a TPU elastomer formulation having enhanced exposure resistance to low temperatures. The TPU is derived from the reaction of 4,4xe2x80x2-methylenebis(phenyl isocyanate), a poly(1,4-butylene azelate), and a chain extender such as an aliphatic diol or bis(2-hydroxyethylether). The elastomer is described as particularly useful in fabrication parts for automobiles and the like which are exposed to severe temperature extremes.
U.S. Pat. No. 4,532,316 discloses polyurethane prepolymers which phase-separate into hard and soft segments. The prepolymers are provided as the reaction productions of a polyol, a diisocyanate, and a low molecular weight chain extender. Preferred diisocyanates include 4,4xe2x80x2-diphenylmethane diisocyanate, cyclohexanediisocyanate, p-phenylene diisocyanate, 1,5-cyclohexane, p-tetramethylxylene diisocyanate, m-tetramethylxylene, and admixtures thereof. Suitable chain extenders include glycols, aromatic diamines, alkanolamines, hydroxy acylamines, and aliphatic and cycloaliphatic diamines, and particularly 1,4-butane diol, 1,6-hexane glycol, hydroquinone di(hydroxyethylether), bis (hydroxyethyl)-bisphenol A, bis(2-hydroxypropyl)bisphenol A, bis(2-hydroxypropyl)isophthalate, bis (2-hydroxyethyl) carbamate, 1,2-bis(2-amino-phenylthio)ethane, trimethylene glycol, di-p-aminobenzoate, resorcinol di(hydroxyethyl)ether, 1,4-cyclohexane dimethanol, 4,4xe2x80x2-dihydroxy diphenyl sulfone, ethanolamine, ethylene diamine, butane diamine, and others. The polyol preferably is selected from poly(akylene ethers), polyesters, polycaprolactones, hydroxyl-terminated polyester amides, polycarbonates, and polybutadiene polyols.
U.S. Pat. No. 3,899,467 discloses polyurethane elastomers which are characterized by high thermostability and thermoplasticity. The elastomers are formed as the reaction product of 3,3xe2x80x2-dimethyldiphenyl4,4xe2x80x2-diisocyanate, a polyester diol such as polycaprolactone diol, and the bis(2-hydroxylethyl ether) of hydroquinone. The elastomers are described to be useful in gaskets, seals, O-rings, wire coating, and the like which require high service temperatures.
U.S. Pat. No. 4,051,111 discloses an injection moldable polyurethane which is described as having a good balance of physical properties including high stiffness and resistance to low temperature impact and elevated heat distortion. The polyurethane is formed as the reaction product of a hydroxyl-terminated polyester or polycaprolactone polyol, an aliphatic diol such as 1,4-butane diol, and a cyclic diisocyanate. Preferred diisocyanates include methylene-bis(4-cyclohexyl isocyanate), isophorone diisocyanate, diphenyl methane-4,4xe2x80x2-diisocyanate, p-phenylene diisocyanate, dichlorodiphenyl methane diisocyanate, dimethyl diphenyl methane diisocyanate, dibenzyl diisocyanate, diphenyl ether diisocyanate, and ditolylene diisocyanate.
U.S. Pat. No. 5,430,121 discloses a TPU which is modified with a segmented siloxane block copolymer. The TPU is formed as the reaction product of a dihydroxy compound such as an alkylene oxide, an isocyanate, a difunctional chain extender, and a linear, hydroxy-terminated polyol which preferably is a polyether or polycarbonate polyol. The chain extender may be an aliphatic diol or cycloaliphatic diamine such as ethylene glycol, butane diol, hexane diol, 1,4-di-(b-hydroxyethyl)-hydroquinone, isophorone diamine, or 4,4xe2x80x2-dicyclohexylmethane diamine.
U.S. Pat. No. 4,521,582 discloses a thermoplasticly-processable, branched TPU which is formed as the reaction product of naphthalene-1,5-diisocyanate, a chain lengthener mixture, and a hydroxyl-terminate prepolymer. The chain lengthener mixture may include diols, triols, and/or polyols such as trimethylol propane, 1,4-butane diol, 1,6-hexane diol, diethylene glycol, neopentyl glycol, ethane diol, and hydroquinone-di-(xcex2-hydroxyethylether). The prepolymer preferably is formed as the reaction product of an isocyanate other than naphthalene-1,5-diisocyanate, such as an aromatic, cycloaliphatic, or aliphatic diisocyanate, and a polyhydroxyl compound such as hydroxyl polycarbonates, hydroxyl caprolactones, hydroxyl-polytetrahydrofurans, or hydroxyl polyethers based on polyethylene oxide and/or polypropylene oxide. The TPU is described as having good dimensional stability under heat, good rebound elasticity even at relatively low temperatures, acceptable low temperature flexibility, and outstanding wear resistance.
The TPUs and methods for processing the same representative of the state of the art have afforded manufacturers a capability to exploit the attractive physical properties of poly-urethanes using economical thermoplastic processing methods. For example, TPU elastomers, as having a desirable combination of such physical properties as good flexibility and resistance to low and elevated temperatures, have found use in the molding of automotive parts which are subject to service temperature extremes. Although a number of formulations have been proposed, PPDI-based systems are often preferred for critical applications as offering an especially attractive convergence of physical properties including low compression set and high resistance to elevated temperatures. Heretofore, however, the injection molding of PPDI-based TPUs generally has been limited to relatively small, annular parts and articles of dimensions less than about xc2xc-inch (6 mm) in cross-section and about 5 inches (12 cm) in diameter. Owing to the excellent physical properties of polyurethanes as compared to other thermoplastics, and particularly to the superior physical properties at elevated temperatures of PPDI-based polyurethanes as compared to other polyurethanes, the capability to fabricate larger part sizes from PPDI-based TPUs has been desired.
It therefore will be appreciated that continued improvements in PPDI-based TPUs would be well-received by the plastics molding industries. A preferred composition would facilitate the injection molding of larger parts and articles than heretofore were available in the art, while retaining the convergence of physical properties which makes PPDI-based formulations preferred for critical service environments such as in automotive, hydraulic, or other high temperature applications.
The present invention is directed to a p-phenylene diisocyanate (PPDI)/polycaprolactone-based thermoplastic polyurethane (TPU) composition having a unique chain extender combination for improved injection moldability. The extender combination includes a crystallinity modifier component which is selected as being effective to alter the crystallinity of the hard segment of the polymer. Such altering reduces the intermolecular hydrogen bonding between the hard segments of adjacent polymer chains, and thereby promotes relative movement between the chains for improved melt flow characteristics.
The improved melt flow of the composition of the present invention facilitates the fabrication of annular parts or other articles of a relatively large size up to about xc2xd-inch (12.5 mm) or more in cross-section and up to about 50 inches (125 cm) in diameter without compromising the superior physical properties inherent in PPDI-based formulations. The inventive composition, moreover, exhibits a relatively high crystallization rate which allows for injection molding cycle times that are 25-50% shorter than for conventional MDI or tolidine diisocyanate (TODI)-based TPU formulations. The composition is moldable into an elastomer exhibiting excellent flex resistance, superior dynamic properties, and good cut and tear resistance even at elevated temperatures. The convergence of such properties makes the composition especially adapted for demanding or critical applications such as automotive parts or hydraulic seals, gaskets, packings, and the like.
It therefore is a feature of the present invention to provide a thermoplastic polyurethane (TPU) elastomer and a p-phenylene diisocyanate (PPDI)/polycaprolactone-based composition therefor having a unique chain extender combination for improved injection moldability. The elastomer is formed as the reaction product of: (A) from about 60 to 80% by weight of a hydroxyl-terminated poly(caprolactone) diol; (B) from about 17 to 22% by weight of a p-phenylene diisocyanate; (C) from about 3 to 10% by weight of a first hydroxyl-functional chain extender; and (D) less than about 2% by weight of a second hydroxyl-functional chain extender different from the first chain extender, the second chain extender being selected as effective to modify the crystallinity of the elastomer.
It is a further feature of the invention to provide a method of injection molding an article from a p-phenylene diisocyanate-based thermoplastic polyurethane elastomer. The method involves forming the elastomer as the reaction product of from about 60 to 80% by weight of a hydroxyl-terminated poly(caprolactone); from about 17 to 22% by weight of a p-phenylene diisocyanate; from about 3 to 10% by weight of a first hydroxyl-functional chain extender; and less than about 2% by weight of a second hydroxyl-functional chain extender different from the first chain extender, the second chain extender being selected as effective to modify the crystallinity of the elastomer. The elastomer so formed is heated to a temperature of from between about 440 to 480xc2x0 F. (225 to 240xc2x0 C.) to form a melt, at least a portion of which then is injected into a mold to form the article.
Advantages of the present invention include the provision of a PPDI-based TPU formulation which facilitates the injection molding of larger-sized parts and articles while retaining superior high temperature performance. Additional advantages include a composition which is moldable into an elastomer exhibiting excellent flex resistance, superior dynamic properties, and good cut and tear resistance at temperatures higher than for conventional TPUs. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein.